The present invention relates to a method for applying multiple-ply wear protection layers having optical properties onto surfaces.
It is conventional to apply layers onto substrates using various techniques, for example laser polymerization, reactive sputtering, or electroplating. U.S. Pat. No. 4,830,873 describes an application of thin transparent layers onto the surface of optical elements by plasma polymerization of silicon initial compounds. The polysiloxane layers thereby created are very hard and transparent. U.S. Pat. No. 4,619,865 describes a method for applying wear protection layers, in particular multiple-ply protective layers, by way of magnetron sputtering. Each layer has a different hardness or abrasion resistance. Different layer properties can also be combined with one another in these multiple-ply layers; for example, one layer acts as a diffusion barrier for water vapor, the second layer is extremely hard, and the third layer can, for example, be electrically insulating.
The problem of imparting sufficient wear resistance to substrates other than steel, ceramic, or glassxe2x80x94especially plasticsxe2x80x94has not, however, hitherto been satisfactorily solved. For example, if plastics are to be mechanically loaded, it is thus necessary, for example, to equip molded plastic parts with wear protection coatings. For this purpose, it is known that in addition to the application of a scratchproof protective varnish layer. It is also possible to apply, for example, wear-resistant polysiloxane layers by plasma polymerization. Often, however, because of the substrate used for example, polycarbonate, it is necessary to make the wear protection layer UV-resistant by way of suitable additives, so that the uppermost plastic layer does not break down when exposed to outdoor weathering and thus degrade the adhesion of the wear protection layer. To prevent this, a UV absorber, for example, is dissolved in the varnish. This is intended, after the varnish has cured, to protect the substrate from the effect of light. Varnishes are not wear-resistant on a long-term basis.
An object of the present invention is to develop a method of combining incorporated or multiple-ply layers that possess optical properties, in particular UV properties such as resistance to and absorption or reflection of UV radiation, with high wear resistance on substrates, in particular substrates that can degrade upon weathering, for example, plastic components.
The present invention provides a method for applying protective layers having optical properties, for example, having good UV protective functions, onto surfaces by way of at least two different deposition steps. One step comprises a plasma-enhanced Chemical Vapor Deposition (xe2x80x9cCVDxe2x80x9d) method, and the at least one other step compromises a material deposition using the Physical Vapor Deposition (xe2x80x9cPVDxe2x80x9d) technique. Combining these methods in one vacuum vessel greatly decreases capital costs. Coating times can be further reduced by operating the two deposition processes simultaneously, which can result in an additional cost decrease. Deposition of the wear protection layer can be performed by plasma polymerization. Plasma polymerization is a coating method in which an evaporable chemical compound, for example, an aliphatic-aromatic or olefinic hydrocarbon compound, preferably methane, ethane, ethylene, or acetylene, is used. It is preferable to use a silicon-organic compound including the elements silicon, carbon, and hydrogen, or with additional nitrogen and/or oxygen, may include tetramethylsilane (TMS), hexamethyldisilane (HMDS), hexamethyldisilazane (HMDS(N)), tetraorthosilicate (TEOS), dimethyldiethoxysilane (DMDEOS), methyltrimethoxysilane (MTMOS), tetramethyldisiloxane (TMDS(O)), divinyltetramethyldisiloxane (VSI2), and very particularly advantageously hexamethyldisiloxane (HMDS(O)). These xe2x80x9cmonomersxe2x80x9dxe2x80x94many of which cannot be polymerized by conventional chemical methodsxe2x80x94can have further evaporable additives mixed into them, for example metalorganic compounds such as tetraethyl orthotitanate (TEOT), tetraisopropyl orthotitanate (TIPOT), or tetrakis(dimethylamino)titanium.
In another embodiment of the present invention, halogenated additives such as tetrafluoromethane, chloroform, or freons can be used. The monomer/additive mixture is then exposed in the vessel to an electric field that can be capable of igniting a plasma of the vapor. The vapor molecules are thereby activated and fragmented into ions and radicals. These highly reactive fragments condense onto the substrate surface and combine into a new, very dense monomer network. In order to prevent the electrodes necessary for the electric field from becoming insulated with insulating layers, which can occur, for example, when silicon-containing vapors are used, it is recommended to power the plasma with electrical high-frequency fields in a frequency range between 50 kHz and 2.45 GHz, preferably between 400 kHz and 2.45 GHz. Because of the higher deposition rate, it is particularly advantageous to use radio-frequency (13.56 MHz) or microwave radiation (2.45 GHz). Microwave plasmas can be operated in pulsed or unpulsed fashion, with or without magnetic field enhancement (ECR). The pulse frequencies can be between a few Hz and 10 kHz, preferably in the range between 50 Hz and 2 kHz; the pulse-to-interpulse ratio can be set without restriction.
In another embodiment of the present invention, in the case of a microwave plasma the chemical structure and stoichiometry of the deposited layer can be more freely adjusted by physically separating the plasma from the deposition region, since fragmentation of the monomer molecules proceeds more gently and more controllably.
In another embodiment of the present invention, this can be achieved by xe2x80x9cremote placement,xe2x80x9d in which a noble gas and/or a non-coating gas, which can be called the xe2x80x9creactive gasxe2x80x9d, e.g., oxygen, nitrogen, hydrogen, ammonia, laughing gas, halogen, is activated and fragmented in the source and/or in the vicinity of the source, and passes by way of a correspondingly guided gas flow toward the substrate. At that point the activated reactive gas encounters the monomer gas, causing the monomer gas to be activated and fragmented. This embodiment of the present invention provides better control of thermal loads, e.g., in the case of microwave-absorbing materials such as plastics, since the source and the point of highest plasma density is located farther away from the substrate.
In a further embodiment of the present invention, deposited polymer networks can be additionally densified by substrate heating and/or ion bombardment. Since additional heating can have limitations in the case of temperature-sensitive substances, e.g., plastic materials, ion bombardment is useful because densification, rearrangement of bonds in the network in the direction of maximum bond saturation of the atoms participating in the network, can be combined with a comparatively small heat input. A further embodiment of the present invention provides using a bias voltage on the substrate by way of an electrode mounted behind the substrate. This allows pulling ions directly onto the substrate. Both pulsed and unpulsed bias voltages can be used in this context. In the case of pulsing, frequencies between 5 kHz and 27 MHz are usable, in particular between 10 kHz and 13.56 MHz. Bias voltages can be used in addition to the PECVD process, i.e., microwave plasma polymerization. In another embodiment of the present invention, bias voltages can also be utilized as the only source for Plasma Enhanced Chemical Vapor Deposition (xe2x80x9cPECVDxe2x80x9d) deposition of the wear protection layer. High-frequency bias voltages have proven particularly effective in this context, in particular those with frequencies in the range of 50 kHz to 800 kHz or 13.56 MHz.
Particular optical properties, in particular UV protection, can be implemented in three ways: first, by way of a package of thin layers having different refractive indices that are capable of reflecting UV light; second, by way of an absorption layer in which the UV light is absorbed and converted into other forms of energy; and third, by the incorporation of atomic or molecular xe2x80x9cclustersxe2x80x9d whose size and concentration in the matrix are selected in such a way that UV radiation can effectively interact with said clusters.
The UV protection function can be implemented by way of metal compounds and, in the case of clusters, also by pure metals. These can be integrated into the wear protection layer as particles, phases, or intermediate layers, e.g. also in the form of multiple-ply layer packages. This is achieved with the aid of a sputtering method, in which there is clamped in front of the cathode a metallic or nonmetallic target from whose surface successive atoms, atom clusters, and particles are evaporated or sputtered by way of ions accelerated in the direction of the cathode. DC magnetron sputtering, pulsed magnetron sputtering, double-cathode sputtering, or high-frequency sputtering can be used for the purpose.
When insulating target materials, e.g., oxides, are used, the use of a bipolar-pulsed double cathode is particularly suitable for integrating the UV protective function in accordance with the present invention. The use of this technique is also particularly advantageous in view of the fact that dielectric wear protection layers created in the PECVD process can also be deposited onto the cathode target, forming there an insulating coating that can gradually reduce the sputtering rate to zero (xe2x80x9ctarget poisoningxe2x80x9d). When bipolar-pulsed double cathodes are operated, this coating can be sputtered off again by the alternating polarity reversal of the individual cathodes, thus preventing any permanent poisoning of the target.
In a further embodiment of the present invention, a hollow cathode is used. Because of the continuous gas flow that is guided out of the hollow cathode, any coating of the target (which is mounted inside the hollow cathode) can be difficult or not possible.
In another embodiment of the present invention, oxides, silicides, carbides, borides, nitrides, sulfides, fluorides, selenides, and tellurides of the elements of the fourth through sixth subgroup, and of the elements La, Ce, Zn, In, Sn, Al, Si, Ge, Sb, Bi, and/or mixtures thereof, are used for sputtering the UV protective layer. As a result, not only metallic but also oxide, silicide, fluoride, selenide, and/or telluride inclusions can be integrated into the layer system.
Deposition by way of a magnetron sputtering technique of the optically active materials that are to be incorporated can, according to the present invention, be accomplished either in that targets made of elements, (e.g., titanium, zinc, or silicon (see above), alloys e.g., Alxe2x80x94Mgxe2x80x94Si or Zrxe2x80x94Al, or corresponding compounds, e.g., TiO, ZnS, SiO, etc., can be atomized with the aid of noble gases e.g., argon, and reacted only later in the PECVD process atmosphere to form the compounds of desired stoichiometry; or sputtering is performed directly with the gas with which the chemical reaction is to take place (the xe2x80x9creactive gasxe2x80x9d), in which context the process must be very accurately controlled, e.g., by way of plasma emission monitoring, because of the risk of poisoning. In this embodiment, a defined post-reaction in the atmosphere of the PECVD process can established.
The post-reaction can be established using process parameters such as the mean free path ratio of the particles between impacts with the reactive gas or vapor molecules, defined by the process pressure, the power input into the PECVD and PVD process, or the target-substrate distance.
Non-coating gases such as, for example, oxygen, hydrogen, diborane, nitrogen, laughing gas, ammonia, nitrogen trifluoride, hydrogen sulfide, and/or fluoride, can be used as reactive gases. Coating gases, for example, hydrocarbons, boron-organic compounds, and halogenated hydrocarbons, can also be used for the post-reaction; they are utilized not as sputtering and reactive gases but as monomers.
In order to establish the chemical composition of the layer accurately and to improve the control of target poisoning, in an embodiment the gas with which the targets are to be sputtered (the xe2x80x9csputtering gasxe2x80x9d) is introduced directly onto the cathodes, while the reaction gas is delivered at a defined distance from the target. This is implemented most easily by way of separate gas supply lines called e.g., gas frames.
In the event that particles are to be incorporated, the size thereof can be established by way of the target/substrate distance and the gas pressure.
A method according to an embodiment of the present invention can be performed in such a way that the two steps are accomplished simultaneously. The result is to create at least one layer having the desired optical and wear-protection properties, in which context the UV protection applied by sputtering can be incorporated homogeneously into the wear protection layer. The chemistry of the incorporated molecules, particles, or plies is influenced by reactions with the reactive gas and/or molecules, fragments, or atoms of the monomer vapor from the PECVD process. Reactions in the deposited layer also exist. For example, sputtered atoms, clusters, or particles of oxidation-sensitive elements or compounds such as aluminum are oxidized, at the latest upon incorporation into a siloxane plasma polymer layer. If a sufficient concentration of monomer vapor is still present in the process gas, the reaction will then, as described, take place even before striking the layer. A central constituent of the method according to the present invention is this mutual chemical influence between the PECVD and PVD methods, which is particularly pronounced when the processes are simultaneous.
Another embodiment of the present invention involves performing facility that is not subdivided, the reactive components of the sputtering process then deriving, as described above, for the most part from the monomer vapor of the plasma polymerization.
Another embodiment of the present invention provides using multiple steps performed in succession (reactive sputtering, plasma polymerization, reactive sputtering, etc.) to more accurately adjust the composition of the layer component that is deposited by sputtering, provided the cathode shutter is closed during the plasma polymerization process so that the cathode targets cannot become coated with plasma polymer. A rather layered distribution of the incorporated species in the matrix can result from the method of the present invention.
In another embodiment of the present invention, a somewhat grater separation between the process gas atmospheres can be achieved by the fact that between the PVD and PECVD processes, the chamber is pumped down, noble gas is admitted, the chamber is pumped down again, and only then is the monomer admitted.
One process and system variant according to the present invention is a vacuum chamber that is equipped with a compartmentalization of the process regions within the vacuum chamber, without airlock technology but with, for example, sheet-metal panels and corresponding management of fresh gas and exhaust gas flows. This also results in a somewhat greater separation of the various processes, allowing controlled adaptation to the desired stoichiometry of the PVD deposition.
In another embodiment of the present invention, a multiple-ply layer structure for the wear protection matrix can also be deliberately selected in order to achieve extreme wear resistance with good homogeneity and excellent UV protection properties, for example, reflection of critical UV wavelengths at the multiple-ply package.
In order to prevent discoloration and maximize reflectivity, layers with an accurately defined thickness, good thickness homogeneity, and alternatingly high and low refractive indices are necessary in this context. The matrix contributes to wear protection with its hardness, and to the interference layer package with its refractive index.
In another embodiment of the present invention, involving soft substrate materials, for example, plastic, a soft matrix base layer is deposited, whose hardness is continuously increased as the layer thickness grows. The individual plies of the reflection layer package are incorporated into this. The overall layer package terminates at the top in an extremely wear-resistant surface layer.
In another embodiment of the present invention, the wear protection matrix can also be deposited in such a way that the adaptation in terms of hardness and elasticity to the substrate and wear load is accomplished in steps. This results in a multiple-ply layer package. In this context, the optical functional material can also be deposited in individual layers.
In the case of a UV-reflective layer package that is to be integrated into a multiple-ply wear protection matrix, it is useful if the individual wear protection layers are optimized not only in terms of their hardness and elasticity, but also in terms of their refractive index. As a result, the wear protection matrix can assume optical functionality in the context of the interference layer package.
In another embodiment of the present invention in which particles are incorporated into the matrix, it is possible to integrate them into the matrix in a concentration, morphology, and/or stoichiometry gradient. For that purpose, the sputtering parameters and/or reactive gas parameters are continuously modified during the PVD process. Especially in combination with a hardness gradient of the wear protection layer, particle incorporation gradients of this kind can be used for optical layers.
The thickness of the combination layer and the concentration of the optical functional materials are established by way of the duration of the deposition process, pressure, flow conditions, and irradiated energy.